Effect of age on physico-chemical properties of the uterine nuclear estrogen receptors of albino rats.

Estrogen receptors (ER) were extracted with high ionic strength buffer from the nuclei of uteri of young (21 weeks) and old (89 weeks) rats. Following the analysis of these receptors on a sucrose gradient and a Sephadex column, two peaks representing the two forms of receptors were obtained. The minor peak sedimented at 6.8 S with Stokes radius (RS) = 7.2 nm, molecular weight (Mr) = 204 K and frictional ratio (f/f0) = 1.71. On the other hand, the major peak sedimented at 4.1 S with RS = 3.3 nm, Mr = 57 K and f/f0 = 1.18. These properties of nuclear ER were similar in both ages. Also, the half life of ER complexes from both ages at 25 degrees C and 37 degrees C were 135 min and 30 min, respectively. However, these complexes were retained for longer periods in the nuclei of young than old rats. Furthermore, the dissociation constant of the binding of nuclear receptors to estrogen remained constant, but the number of binding sites decreased from 1.56 in young to 1.05 pmol/mg DNA in the old. In young rats, about 61% of nuclear receptors bound to DNA-cellulose. Out of this 2/5 was eluted with 0.15 M and the remaining 3/5 with 0.5 M KCl. On contrary, only 37% of total receptors bound to DNA-cellulose in the old. Out of this 3/5 was eluted with 0.15 M and the remaining 2/5 with 0.5 M KCl. These data suggested that despite the similarity in different physicochemical properties, the number of estrogen binding sites and the retention time of ER complexes in nuclei and the ability of these complexes to bind to DNA decrease in the uterus of old age.